AbstractThe membrane constants of the slow muscle fibres from the Atlantic hagfish were determined by analyzing the responses to sinusoidal currents at different frequencies. The apparent low frequency membrane resistance was 20–50 kΩcm2and was not significantly altered by replacing external chloride with methyl sulphate. The low frequency capacitance was 2.1–2.4 μF/cm2and decreased to about 1 μF/cm2at 100 Hz. A model of the T‐system consisting of a number unbranched T‐tubules was fitted to the impedance locus. About 15 tubules per sarcomere with a length approximately 8 times the fibre radius gave good fit to the observed impedance locus. The electrical characteristics of the T‐system allow an efficient electrotonic spread of the junction potential through the fibre c